Radiometric limit switches are usually applied, when conventional limit switches are not applicable due to especially rough conditions at the measuring location. Very frequently, extremely high temperatures and pressures reign e.g. at the measuring location or chemically and/or mechanically very aggressive environmental influences are present, which make the use of other measuring methods impossible.
Radiometric limit switches are applied, for example, for monitoring an exceeding or subceeding of a predetermined fill level of a fill substance in a container. They serve the purpose, for example, of protecting against overfilling or running empty.
To this end, they involve a radioactive radiator arranged externally on the container. During operation, the radiator sends radioactive radiation along a radiation path extending at the height of the limit level to be monitored through the container. Externally on the oppositely lying side of the container, there is arranged, applied terminally in the radiation path, a detector, which quantitatively registers the radiation intensity emerging from the container. The emerging radiation intensity depends on the geometric arrangement and on absorption. The latter is dependent on the fill level of the fill substance in the container and on density.
Correspondingly, the radiation intensity has a minimum dependent on the density of the fill substance, when the fill level lies above the radiation path. Conversely, the radiation intensity has a maximum, when the fill level lies below the radiation path.
The minimum and the maximum radiation intensities are determined today regularly in a calibration method performed by the user in the context of start-up of the radioactive limit switch. In such case, the two above mentioned conditions of filling, free of medium and covered by medium, are established in the container, and the associated minimum and maximum radiation intensities are measured by the limit switch.
Based on the minimum and the maximum radiation intensities, for example, a threshold value, usually referred to as the switching point, is established for the radiation intensity. An exceeding of the threshold value corresponds to a change into the state, free of medium, and its subceeding to a change into the state, covered. In the subsequent operation of the limit switch, via a comparison of the radiation intensity measured with the threshold value, it is detected whether the limit-level to be monitored has been exceeded or subceeded. Frequently, for achieving an as flutter free as possible, stable, switching behavior, a switching hysteresis is provided. For this, based on the minimum and the maximum radiation intensities, lower and upper threshold values are established. In operation in this case, a state change into the state, free of medium, is only displayed, when the radiation intensity measured exceeds the upper threshold value, and a state change into the state, covered by medium, is only displayed, when the radiation intensity measured subceeds the lower threshold value. In between the two threshold values, the limit switch continues to show the last ascertained state. Calibration methods require today regularly the aid of the user in bringing about the required states, and the limit switch must display the presence of the respective state, in order that the associated maximum and minimum radiation intensities can be measured and stored associated with the respective states. Accompanying the calibration of the two filling conditions, free of medium, and covered with medium, is, as a rule, an interference with, interruption of and/or delay of the manufacturing- and/or processing method running at the measuring location.